Recently it was demonstrated that elevated levels of serum high density lipoproteins (HDL) play an important role in prevention of coronary heart disease (CHD). Dietary perturbations such as a diet extremely rich in carbohydrates (80-90% of calories) or one with a high P/S ratio (of 4.0) have been shown to cause a decrease of HDL levels. The diets usually recommended for the prevention of CHD or treatment of hyperlipoproteinemia have an increased carbohydrate content and P/S ratio. These diets may therefore decrease the HDL levels. Thus it is proposed to investigate the effect of these diets on HDL levels and composition, as well as changes in the other lipoproteins. The experimental design includes three different study populations: a) healthy volunteers in whom two dietary interchanges will be performed: 1) an isocaloric diet with constant P/S ratio of 1.0, whose fat content of 30% in which the P/S ratio will be varied from 0.4 to 1.5; b) A group of patients with hyperlipoproteinemia type IIa and type IV on ad-libitum diet as compared to type-specific recommended diets; c) Obese patients following a weight-reduction dietary regime. In each subject, the levels and composition of the plasma lipoproteins and of the HDL subpopulation (HDL2 and HDL3) will be determined. HDL2 and HDL3 will be further separated by zonal ultracentrifugation and be fully characterized. This will include determination of hydrated densities, lipid composition, apoprotein profile and morphology. In addition, plasma and lipoprotein apo A-I apo AII, apo-B and apo-E levels will be determined by radioimmunoassay. Finally, the effects of the different dietary regimes in the various subject groups on the synthesis, catabolism and distribution of the major HDL apoproteins apo AI and apo A-II) will be determined by the turnover technique. The study thus will elucidate the effects of the above-mentioned diets lipoproteins and their major apoproteins.